In order to control an induction machine using a frequency converter, a commonly used control technique is ‘vector control’, also known as Field Oriented Control (FOC), which makes use of knowledge of the machine speed and angle to determine the appropriate amplitude and angle (phase) of converter output voltage that will allow independent control of machine flux and torque or power. The machine shaft speed and angle can be measured directly, such as via the use of an encoder, or indirectly using an ‘encoderless’ speed observer.
Many known controllers relate to applications where the induction machine is used as a motor as well as a generator. Therefore such controllers need to cater for conditions that may not be relevant in a pure generator application. Such condition may include operation at and around zero speed, and therefore at and around zero volts on the generator's stator terminals. Motoring applications may also typically require knowledge of the shaft speed and/or angle in addition to the angle of the rotor flux. These requirements can lead to complicated control strategies.